Scanning device for a laser beam focus

ABSTRACT

The focal distance (f) of a laser ( 1 ) beam ( 2 ) is adjusted by projecting the beam ( 2 ) onto the center of a mirror ( 3 ) of variable curvature and controlled by a depression circuit or by electrostatic forces. Thus the focus ( 4 ) can be displaced in a direction which is inaccessible to the usual means such as turning but rigid mirrors.

The invention relates to a device for sweeping a laser beam focus.

Power lasers make it possible to focus high luminous energy in a smallvolume focus and can be applied in physics to the study of particles,plasma confinement, machining, or surgical organic tissue section. Mostof these applications involve moving the focus so that it sweeps acrossa trajectory, which is difficult to achieve in a simple way. Since thelaser itself is voluminous, it is preferable to leave it immobile and tomove the beam only. This is possible by passing it along an optic fibre,but not in all cases, and in particular not outside frequencies beyondthe visible spectrum, whereas infrared lasers are greatly appreciatedbecause of their power. Another means of displacing a beam consists ofpassing it by using a turning mirror, but movement of the focus in alldirections can only be obtained by using a succession of mirrors turningaround different axes, which poses considerable problems for couplingthe position of the mirrors, since a rotation of one mirror necessitatesdisplacing all the downstream mirrors so that the beam continues to passbetween them. Furthermore, a large part of the energy is finally lost.

This is the reason why it is thus proposed to sweep a trajectory for alaser beam focus without moving the beam, but by modifying its focaldistance. This is achieved by means of a variable curvature mirror,whose curvature variation is associated with a variation in the focusingof light beams; the flexibility of the membrane allows significantdeformations and thus makes long sweepings possible. It should be notedthat such mirrors are already known to those skilled in the art intelescopes or other optical devices, but herein the variation of focallength was used in particular to adjust the apparatus and to provide aclear image of a natural radiation (see French patent 2 662 512). Thedeformation is often controlled by networks of piezoelectric sensorswhose displacements determine a two-dimensional profile of the membrane,but such actuators have only a small displacement range and theirsimultaneous control of different displacements is costly. They arepoorly adapted for imposing regular curvatures (rather than localdisplacements), especially if the membrane is submitted to big overalldisplacements. An example of such prior art is given in U.S. Pat. No.4,934,803-A.

To resume, the invention relates, in its most general form, to a devicefor sweeping a laser beam focus, comprising a flexible and stretchedmembrane onto which the beam is projected and reflected upstream fromthe focus, adjustable means for varying the curvature of the membraneand means for cooling the membrane. Greater possibilities for adjustingthe focusing can be obtained if the means for varying the curvature aredouble and comprise a means for applying pressure distributed over themembrane and a means for applying pressure concentrated on the membrane.The means for distributed pressure application is a means for varyingpressure from a closed chamber defined by the membrane, and the meansfor concentrated pressure application is an electrical circuit with adirect voltage generator between the membrane and an electrode adjacentto a central part of the membrane.

The means for cooling the membrane are essential for evacuating theincident heat from the laser. They can comprise means for blowing gasacross a face of the membrane, and these means can consist of a circuitfor renewal of the gaseous contents of the closed chamber.

Finally, devices for inclination of the membrane can allow adjustment ofthe orientation of the mirror and therefore the position of the focus ofthe beam in various directions.

The invention will now be described in relation to the following figureswhich show a preferred embodiment:

FIG. 1 is a general view of the device,

FIG. 2 shows the mirror, and

FIG. 3 is a curve giving the experimental results.

FIG. 1 shows a laser 1 which projects a beam 2 towards a mirror 3 whichreflects it and focuses it; the focus of the beam 2 is given thereference 4. The mirror 3 is a flexible membrane of small thickness,which can be made of metal, ceramic or composite material, possiblycovered with a reflecting coating 5 on the face opposite the beam 2 ifthe base material itself is not so. Foil, metal-coated carbon, andalumina based ceramics can be suggested as examples, noting that thechoice often depends on the wavelength of the reflected light. Themembrane is stretched over a drum 6 which defines with it a base wall 7of a closed chamber 8. Means described below and present mainly in theclosed chamber 8 make it possible to modify the curvature of the mirror3 and thus the distance of the focus 4, which sweeps the direction X thesame as that of the beam 2 downstream from the mirror 3. Since the beam2 is projected towards the centre of the membrane 3, whose averageorientation does not vary, it is not deviated when the curvature of themirror 3 is modified, but a deviation of the beam 2 in a perpendiculardirection Y is possible by making the mirror 3 turn: the drum 6 is thusmounted on a rotational axle 9 passing through the centre of the mirror3 and perpendicular to the direction Y. The axle 9 is driven by a motor,not shown. It is also possible to displace the beam 2 according to athird direction Z by mounting the mirror on a universal joint. There isno need to go into detail about such devices, which are known to thoseskilled in the art, for orienting plane mirrors, and therefore FIG. 2 isnow described.

A delivery pipe 10 and a suction pipe 11 of a pump 12 connected to acold source 13 open into the closed chamber 8; the air in the closedchamber 8 is recycled by the pump 12 where it is cooled, which maintainsthe contents of the closed chamber 8 together with the mirror 3, heatednonetheless by the beam 2, at an acceptable temperature. The pump 12operates in a closed circuit and therefore has no influence on thepressure in the closed chamber 8. The same cannot be said of a secondpump 14 which is adjustable and opens to the exterior in order towithdraw part of the contents of the closed chamber 8 and to submit itto a depression leading to indentation of the mirror 3. The adjustmentof the depression by the pump 14 varies the curvature of the membrane,the focusing of the beam 2 and the position of the focus 4.

An analogous effect is obtained with an electrode 15 located oppositethe centre of the membrane and which penetrates into the closed chamber8 passing through the base wall 7. An electrical circuit 16 connects theelectrode 15 to the mirror 3 through the intermediary of the base wall 7and the drum 6. The electrical circuit 16 comprises a direct voltagegenerator 17, a rheostat 18 to vary the voltage applied to the terminalsof the circuit, and a sine-wave voltage generator 19 in series with theformer and which can be started or stopped by a switch, not shown. Theelectrostatic charges of opposite signs created by the generator 17between the membrane and the electrode 15 also produce an attraction andcurvature of the membrane. In practice, one can use either one or theother of these systems, but it should be noted that if the depression ofthe pump 14 is applied uniformly over the whole surface area of themembrane, the electrostatic forces tend to be concentrated in the centreof the membrane, which can produce a different deformation; depending onthe case, one or other of these methods may be preferable or acombination of the two to obtain the best results. The means with thepump 14, which controls a regular curvature, will generally be preferredfor providing more precise results, since the focusing will beindependent of the place of reflection of the beam 2, but the electrode15 can provide stronger localised focusing and may be preferred forextreme states of deformation of the mirror 3.

Another advantage of the double control means, especially with lowinertia electrostatic control, is given below. The rheostat 18 makes itpossible to vary the electrostatic attraction. Operation of thesine-wave voltage generator 19 produces a periodic variation of thequantity of electric charges and therefore oscillation of the mirror 3,welcome in certain applications such as welding thick parts, where thefocus 4 moves to different depths of the joint.

The electrode 15 can be thin or, on the contrary, widened out into aplate 20, depending on whether one wishes to concentrate or distributethe forces evenly on the surface of the mirror 3; it can be hollow andact as an extension of one of the delivery and suction pipes 10 and 11;when the plate 20 exists, this is also hollow and provided with holes 21for the passage of gas along its front face, which directs anddistributes the cooling circuit efficiently onto the rear face of themirror 3.

FIG. 3 shows the variations of a focal distance f in function of adepression P expressed in Napierian logarithms. It can be seen that theamplitude of the focal distance f, which can be applied between a nulldepression and a minimum asymptotic distance obtained at highdepressions, is very high in relative value as in absolute value; thefocal distance f is expressed in meters and was obtained with a drum of30 mm diameter and a membrane of 0.5 mm thickness. Thinner membranescould make it possible to reduce the minimum focal distance even more.

The pressure or blowing of gas to create the curvature or the coolingcan be applied to the external face of the mirror 3. It is possible tofocus the beam 2 in a line instead of a point if the mirror 3 islengthened and stretched between the two long sides of a rectangularframe replacing the circular drum 6.

The mirror 3 can be given other geometric shapes in function of theresult to be obtained.

In the same way, it can be of variable thickness, for example thinner atthe centre: the radius of curvature determining the focusing of the beam2 is then smaller without the mechanical resistance of the mirror 3being greatly affected.

Since the beam 2 generally only reaches a relatively small centralportion of the mirror 3, whose curvature is almost uniform, the focusingis nearly perfect.

Other cooling circuits are possible as well as other fluids, and inparticular liquids.

Finally, other means for deforming the mirror 3 can be envisaged, butmeans without solid contact are preferable and probably necessary forthe curvature to be regular and without any significant coupling betweenthe two means, whose actions can be adjusted independently and worktogether without any problems for the operator. Furthermore, thecurvature of the mirror 3, which controls the focusing, cannot becontrolled directly except through force fields or fluids, whereas solidactivators generally control displacements.

What is claimed is:
 1. Device for sweeping a laser beam focus,comprising a flexible and stretched membrane (3) onto which the beam (2)is projected and reflected upstream from the focus (4), adjustable meansfor varying the curvature of the membrane and means for cooling themembrane, characterised in that said means for varying the curvature aredouble and comprise means for applying pressure distributed over themembrane and means for applying pressure concentrated on the membrane,and the means for distributed pressure application is a means (14) forvarying the pressure in a closed chamber (8) defined by the membrane,and the means for concentrated pressure application is an electricalcircuit (16) with a direct voltage generator (17) between the membrane(3) and an electrode (15) adjacent to a central part of the membrane. 2.Device for sweeping a laser beam focus according to claim 1,characterised in that the electrical circuit further comprises anoscillating voltage generator between the membrane and the electrode. 3.Device for sweeping a laser beam focus according to claim 1,characterised in that the means for cooling the membrane comprise meansfor blowing gas in front of one face of the membrane.
 4. Device forsweeping a laser beam focus according to claim 3, characterised in thatthe means for blowing gas comprise a circuit (10, 11, 12) for renewingthe gaseous contents of the closed chamber (8).
 5. Device for sweeping alaser beam focus according to claim 1, characterised in that itcomprises a device (9) for orientation of the membrane.